To start things off, I'll repeat a few comments I made in a Facebook group:
Some thoughts after a first quick read-through:
In general, my first impressions are good and I will definitely be buying the Companion and the future installments. The book is, as usual, very pretty and the layout of the adventure a bit less confusing than the original.
There wasn’t really a ton of new or changed stuff, but that’s not surprising – while perhaps not the best installment of the campaign, Shadows was always the most solid and complete as it was. It didn’t need a lot more.
I especially enjoyed the “Grognard boxes”, offering suggestions on how to change NPCs, scenes and/or even plot points around. While their ostensible purpose is to make the adventure fresh for players who’ve played the old version, I think they might actually be quite helpful for a more inexperienced GM as examples of how a published adventure might be tweaked to fit their taste. I know I would have found something like this very illuminating in younger days.
What we don’t get, and what I would very much have liked, is a proper outline or synopsis of the whole campaign. In the introduction we only get the barest hints that don’t really tell us anything we didn’t know – DotR will be structured around river travel, PBtT will be set in Middenheim, The Horned Rat will feature the Skaven, and Empire in Ruins will be about war and politics and at least partially set in Altdorf. As a GM already some way through my own modified run of the campaign, it would have been very helpful to have at least a rough idea of where they were going with things, so I could have an idea which of the new elements were stuff I’d be interested in using and thus should start foreshadowing. But on the other hand, I understand they presumably want to keep the punters hungry and curious, and I also suppose my preferences here aren’t typical of the target audience. All fine and good, as long as they actually get the rest of the damn books out .

What we do get is a number of new scenes foreshadowing future developments, some of them quite tantalizing. These are at least, helpfully, clearly labeled as such. I might start another thread about these hints and speculation on what they might be leading up to.
There’s also tons more background detail here and there (notably in the Bogenhafen appendix), some of which seems very useful and some of which seems quite redundant.